NGC 3628, Melissa J. Haegert
NGC 3628 has had a colorful history.
This galaxy is found among a group of three galaxies known as the Leo Triplet, located in the constellation Leo. NGC 3628's edge-on orientation yields a glorious view of its "dust lane," the visible band of dust blocking the light from the central bulge of the galaxy. This reduction of the light makes NGC 3628 the faintest galaxy in the Leo Triplet, but of the three galaxies, the story told by NGC 3628 is perhaps the most fascinating. Careful inspection of NGC 3628's central bulge itself reveals a faint but distinguishable "X-pattern" symmetric around the galaxy's minor axis; this enigmatic X-shape reveals hidden details about NGC 3628's past (see below).
Charles Messier — the famous 18th century astronomer who published the Messier catalogue of fuzzy objects in the night sky — skipped over NGC 3628, giving Messier numbers to the galaxy's two neighbors (M65 and M66) instead. Four years after Messier discovered these two, in 1784, astronomer William Herschel turned his own telescope back to this region to discover NGC 3628.
According to most sources, the Leo Triplet lies about 35,000,000 light years away. At this distance, its angular size of 13.1 arcminutes gives it a diameter of about 133,000 light years (40.9 kpc), which is about four-fifths the diameter of the Milky Way. (In 1988, Haynes et al. used a Tully-Fisher calculation to approximate the galaxy's distance to be 25,000,000 ly; this would yield a diameter of 95,300 ly, about three-fifths the diameter of the Milky Way.)
The above image was taken at the US Naval Observatory's 40-inch Ritchey Chrétien Aplanatic Telescope, located in Flagstaff, AZ. The image covers an area of sky about 13 x 16 square arcminutes. The USNO telescope has a wider field of view than the telescope at the Calvin-Rehoboth Observatory, but it still leaves out interesting details about NGC 3628.
Image credits: Steve Mandel's galaxyimages.com (left) NASA's Astronomy Picture of the Day (right)
A wider field of view effectively takes “a step back” to look at the big picture of what is happening to NGC 3628. The images above show NGC 3628 among its group of galaxies, and reveal an elaborate tail streaming off of the Triplet's faintest member. This phenomenon is known as a “tidal tail,” and it occurs when galaxies interact with each other. The mutual gravity of two or more interacting galaxies will often strip stars away from one or both of the galaxies, creating beautiful tails like the one seen above.
Neutral hydrogen reveals interactions
The best way to see the interactions between the members of the Leo Triplet is to look at the light emitted by these galaxies in different wavelengths. In particular, the "HI line," also known as the “21-cm” line because of its wavelength, reveals the emission of light from neutral hydrogen gas. As light hits this gas, it gives the hydrogen atoms enough energy to cause their electrons to jump into their excited state. When these excited electron orbits decay back down, the atoms emit photons at 21-cm wavelengths. This emission, invisible in optical images, shows the pattern of interactions between the three galaxies.
The above image, created by Haynes et al. in 1979, reveals the contours of emission from neutral hydrogen. NGC 3628, located at the top, clearly displays its streaming tail, and the contour plot even shows material in between the trio.
The above image (Arp et al. 2002) has used the neutral hydrogen contour plot from the previous image, layering it over a more recent image. The study that used this image sought to demonstrate that the proximity of several quasars (marked with X's on the plot) to NGC 3628 is an indication that quasars are associated with ejection activity from galaxies. This hypothesis has been much debated recently in the astrophysics community.
NGC 3628's unusual X-ray object
Not only have astronomers been looking at neutral hydrogen in NGC 3628, but they have also been imaging the galaxy in the X-ray spectrum. In 1991, X-ray images of NGC 3628 revealed an “intermediate-luminosity X-ray object” (IXO) that was close to, but not directly on top of, the nucleus of the galaxy (Strickland et al. 2001). This IXO is even more interesting because it is a “strongly variable" source. Between December 1991 and March 1994, it faded by at least a factor of 27, effectively disappearing from sight. In the late 1990s and early 2000s, it reappeared again, to be imaged by Harvard University's Chandra X-ray Observatory (see figure).
The image on the left was taken in the X-ray spectrum by the Chandra X-ray Observatory. The image zooms in on the central nucleus of the galaxy, approximately on the scale shown in the visual image. The nucleus is marked in the X-ray image by an X; the IXO is labeled to the lower right. Chandra image credit: Strickland et al. (2001). Image combined with visual by M. Haegert.
According to Strickland et al. (2001), several of these “enigmatic” objects exist, and have been classed together based on their similar characteristics: slightly off-center from the galactic nucleus, much more luminous than normal X-ray binaries, but less luminous than active galactic nuclei (AGN, nuclei that are highly luminous due to the accretion of mass into a black hole). Strickland et al. offers one explanation: these strange objects are “intermediate-mass” black holes — not large enough to be AGN, which reside in the exact center of their galaxies, but still large enough to exhibit some of the characteristics of AGN, such as luminous X-ray emission. In the end, however, Strickland et al. concedes that the multiplicity of possibilities — intermediate-mass black hole, normal X-ray binary, or young and X-ray luminous supernova remnant — leave this IXO's identity indistinct.
Dust in the plane
As is clearly evident in all optical images, NGC 3628 has a glorious dust band blocking a great deal of its light in the line of sight. With image processing software, it is possible to measure the amount of light that has been blocked — the “dust extinction” of NGC 3628 — and compare that extinction to predictions for galactic dust.
The image taken with the USNO telescope was made using light in the red (R), green or visual (V), and infrared (I) parts of the spectrum. Because infrared waves are longer in wavelength, they can travel through the dust in the galaxy, so the infrared light (shown as the color red in the image above and below) is much less extincted than the green or red light. This favorable transmission of redder light through dust is known as “reddening,” and it is visible in the image in the reddish color of the galaxy's dust lane.
The white line in the above figure shows the minor axis of the galaxy, the axis along which dust extinction is most easily measured (the colored lines will be referenced later).
The graphs show the dust extinction for both the green (above) and infrared (below) light. In each graph, the blue trace measures the amount of pixel counts (effectively, the brightness) of the upper half of the galaxy, while the pink trace measures the amount of pixel counts in the lower half of the galaxy. As is evident in the image, most of the dust extinction occurs in the lower half of the galaxy.
In both of the above graphs, residuals obtained from subtracting the lower half from the upper half (at a projected radius of about 0.7 kpc from the center of the galaxy, a radius at which the upper half is not yet affected by dust) yield the dust extinction in a ratio of brightnesses. This can be converted to a difference in magnitudes, which yields the dust absorption for each band of the spectrum: R, V, and I. The difference in this absorption yields a calculation of the “reddening” of the galaxy's color at that particular radius. For NGC 3628, the V-R reddening is 0.07 magnitudes, and the R-I reddening is 0.08 magnitudes.
The “optical depth,” known as τ, in each part of the spectrum, also follows from the absorption. If the dust is “optically thin,” the value of τ will be small, and the light coming through will not be as absorbed. On the other hand, if the dust is “optically thick,” the value of τ will be large, and much of the light will be absorbed. For NGC 3628, this calculation yielded values of τ(V) = 1.70, τ(R) = 1.64, and τ(I) = 1.57. Not surprisingly, this indicates that the dust is much more optically thin in the infrared, and lets through a great deal more light.
NGC 3628's "X" pattern
Our USNO image displayed a faint X pattern across NGC 3628's central bulge, and brightness profiles taken along each of the colored lines in the above image revealed that the central bulge has a very distinct — and unusual — shape.
The graphs below show the results for each of the line profiles for each filter. The green trace represents the green filter, the red trace is the red, and the black is the infrared. In the graph for line 1 (the red line in the image above), the central bright peak of the foreground star has been subtracted, but in the remaining data the peaks on the edges that make up the lines of the X can be seen. Moving down closer to the plane of the galaxy, in line 2 (green in the image), we see the distinctively flat shape of the light profile, indicating that this is not a typical galaxy bulge, which would show a peak in brightness at the center that drops off exponentially on either side. Closest to the midplane, line 3 (blue) shows this expected trend.
In interpreting this unusual X pattern, we have speculated that NGC 3628 is actually a barred spiral galaxy — a type of spiral galaxy that has a bar running through its center, inside of its spiral arms — and our line of sight places us in position to view the bar end-on, sighting down the length of it. This end-on orientation seems the most likely, given the small size of the X relative to the bright disk of the galaxy, though from this vantage point it is difficult to tell the precise orientation without many assumptions.
The idea that NGC 3628 might have a bar is not an original one; in a 1998 paper studying supernova remnants in the galaxy, Cole et al. mentions the X-pattern as a side point, and offers possible explanations for its appearance, saying that there has been “much discussion” as to its origin. According to a 1988 paper by Whitmore & Bell, this structure might indicate that NGC 3628 is the “missing link” between two types of galaxies: polar-ring galaxies and “peanut-shaped” galaxies. Whitmore & Bell also analyze the creation of X-structures with thought to tidal interactions between colliding galaxies — a category which is certainly appropriate for NGC 3628.
In a more recent paper on X-structures in galaxies similar to NGC 3628, Mihos et al. (1995) did numerical simulations to show the structure of disk galaxies upon collision with low-mass galaxies. Their results showed that this interaction forms a bar in the main disk galaxy, and after the smaller galaxy has merged, the bar “buckles vertically … sending disk material well out of the disk plane.” This creates “an X-shaped feature when seen edge-on.” This seems to be an accurate description of NGC 3628's history, given its evident interactions with other galaxies in the past and present.
Curiously, NGC 3628's published galaxy classification makes no mention of the possible — or arguably even highly likely — presence of a bar. The galaxy is classified as a type "Sbc D" in one database (SIMBAD), and a type "Sb pec sp HII LINER" in another database (NED). This cryptic string of numbers tells astronomers about NGC 3628's characteristics relative to other galaxies. The “S” indicates that the galaxy is a spiral, and the “b” or “bc” indicates that the galaxy has more of a tight bulge with more open arms. The “pec” denotes the fact that NGC 3628 is a “peculiar” galaxy, due to its recent interactions with other galaxies. The “HII LINER” indicates that spectral analysis of NGC 3628 shows that the galaxy is slightly active (not on the level of an AGN, though).According to Mihos et al.'s numerical simulations, which explain the visible X-structure in terms of a bar created from tidal interactions, it would seem most appropriate to re-classify NGC 3628 to include the presence of a bar (type “SB”).
Cole, G. H. J., C. G. Mundell, and A. Pedlar. "Neutral hydrogen absorption observations of the central region of NGC 3628." Monthly Notices of the Royal Astronomical Society. 300: 656-664. 1998. <http://adsabs.harvard.edu/abs/1998MNRAS.300..656C>.
Elmegreen, Debra M. Galaxies and Galactic Structure. New Jersey: Prentice Hall, 1998.
Frommert, Hartmut and Kronberg, Christine. "NGC 3628." Students for the Exploration and Development of Space. <http://seds.lpl.arizona.edu/messier/xtra/ngc/n3628.html>
Haynes, Martha P., Riccardo Giovanelli, and Morton S. Roberts. "A Detailed Examination of the Neutral Hydrogen Distribution in the Leo Triplet NGC 3623, 3627, and 3628." The Astrophysical Journal. 229: 83-90. 1979. <http://adsabs.harvard.edu/abs/1979ApJ...229...83H>.
Kutner, Marc L. Astronomy: A Physical Perspective. New York: Cambridge University Press, 2003.
Mihos et al. "A Merger Origin for X Structures in S0 Galaxies." The Astrophysical Journal. 447: L87-L90. 1995. <http://adsabs.harvard.edu/abs/1995ApJ...447L..87M>.
Strickland et al. "Another Intermediate-Mass Black Hole in a Starburst Galaxy? The Luminous X-ray Source in NGC 3628 Reappears." The Astrophysical Journal. 560: 707-714. 2001. <http://adsabs.harvard.edu/abs/2001ApJ...560..707S>.
Whitmore, Bradley C. and Marylin Bell. "IC 4767 (The "X-Galaxy"): The Missing Link for Understanding Galaxies with Peanut-Shaped Bulges?." The Astrophysical Journal. 324: 741-748. 1988. <http://adsabs.harvard.edu/abs/1988ApJ...324..741W>.
This research has also made use of the NASA/IPAC Extragalactic Database (NED) which is operated by California Institute of Technology, under contract with the National Aeronautics and Space Administration. Also used was the SIMBAD Astronomical Database operated at CDS, Strasbourg, France.
The raw images were calibrated using median-combined bias files and flat files only. (The images were taken at about -99 degrees Celsius; thus dark subtraction was unnecessary.) Some scattered light was present in the image that was not present in the flat; this was responsible for several "doughnut" shaped artifacts in the final image. The image shown on this web page has had several of these artifacts removed for presentation, but all scientific analysis was done on the original images, with careful avoidance of any artifacts.
The calibrated images were then combined by filter, and color-combined using MaxIm DL 4. The infrared image is displayed as "Red," the red image as "Green," and the green image as "Blue," with balance values of 1, 1.3, and 1.3, respectively. The image was not color saturated. Finally, the image was converted to JPEG format using a Gamma value of 0.6, a maximum of 1000, and minimum of 490.
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